Georg Dresen

Acoustic emission and velocities associated with the formation of compaction bands in sandstone

Received 31 May 2005; revised 2 May 2006; accepted 22 June 2006; published 7 October 2006. [1] A series of laboratory experiments has been conducted in which three-dimensional (3-D) locations of acoustic emissions (AE) were recorded and used to analyze the development of compaction bands in Bleurswiller sandstone, which has a porosity of 25%. Results were obtained for saturated samples deformed under triaxial compression at three different confining pressures (60, 80, and 100 MPa), a pore pressure of 10 MPa, and room temperature. We recorded acoustic emissions, compressional and shear wave velocities, and porosity reduction under hydrostatic condition and under triaxial loading conditions at a constant axial strain rate. Our results show that seismic velocities and their amplitude increased during hydrostatic pressure build up and during initial axial loading. During shear-enhanced compaction, axial and radial velocities decreased progressively, indicating an increase of stress-induced damage in the rock. In experiments performed at confining pressures of 80 and 100 MPa during triaxial loading, acoustic emissions were localized in clusters. During progressive loading, AE clusters grow horizontally, perpendicular to the maximum principal stress direction, indicating formation of compaction bands throughout the specimens. Microstructural analysis of deformed specimens confirmed a spatial correspondence of AE clusters and compaction bands. For the experiment performed at a confining pressure of 60 MPa, AE locations and microstructural observations show symmetric compaction bands inclined to the cylinder axis of the specimen, in agreement with predictions from recent theoretical models.

Fracture process zone in granite

In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 μm/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is ∼9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be ∼2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

Acoustic emission, microstructure, and damage model of dry and wet sandstone stressed to failure

Twenty-three uniaxial compression tests were performed on dry and wet Flechtingen sandstone from Germany. Compressive strength of wet core is 60% of the strength of dry core. Before fracture, the transverse P wave speed drops by 13% and the pulse amplitude by 22% for wet and 37% for dry cores. Accumulated strain energy doubles for dry core. Acoustic emissions (AE) are detected with 10 sensors for 19 cores. AE activity starts at 84% of the fracture strength of wet cores (55 MPa) and at 91% of the strength of dry cores (87 MPa). The ratio of located to recorded AE is 0.37 for dry and 0.13 for fully wet cores. AE hypocenter patterns document the development of two opposite fracture cones. The negative slope of cumulative AE-amplitude frequency distribution drops by 50% before failure in dry cores. The slope of the wet core drops and recovers. Energy discrimination of AE detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture. Using the analogy to visible light microfracturing events are separated into high-energy short pulses (blue AE) and low-energy pulses with long duration times (red AE). Blue AE are explained by intragranular grain breakage, red AE by multiple stick slip on crack planes or grain boundaries. Deformed cores show highly fractured calcite cement and mostly intact quartz grains. The stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral (calcite).

Scaling and universality in rock fracture.

We present a detailed statistical analysis of acoustic emission time series from laboratory rock fracture obtained from different experiments on different materials including acoustic emission controlled triaxial fracture and punch-through tests. In all considered cases, the waiting time distribution can be described by a unique scaling function indicating its universality. This scaling function is even indistinguishable from that for earthquakes suggesting its general validity for fracture processes independent of time, space, and magnitude scales.

Grain boundary diffusion creep of synthetic anorthite aggregates: The effect of water

To investigate the effect of trace amounts of water on plastic deformation of feldspar, we fabricated synthetic polycrystalline aggregates of pure anorthite from a glass. The glass powder was either densified and crystallized at 1473 K and 0.1 MPa or hot isostatically pressed at 1443 K and 300 MPa confining pressure. Fourier transform infrared spectrometry indicates a water content of 0.002–0.0035 wt % (300–550 H per 106 Si) for specimens prepared at atmospheric pressure. Hot-isostatically pressed samples contain 0.05 wt % to 0.1 wt % (8000–15000 H per 106 Si), depending on whether they were crystallized from glass powder predried at 1073 K for 2–3 days or from glass powder as received. In the wet samples, <1 vol % glass was found. Transmission electron microscopy showed that two-grain boundaries are glass-free to a resolution of 5 nm. We performed creep experiments at 0.1 MPa pressure with temperatures ranging from 1373 to 1573 K and stresses ranging from 1 to 100 MPa. The mechanical data indicate grain boundary diffusion controlled creep with a stress exponent n = 1.0±0.1 and a grain size exponent m = −2.9±0.2. The activation energy for creep is 585±45 kJ mol−1 or 377±38 kJ mol−1 for dry or wet specimens, respectively. If extrapolated to natural temperatures (≅973–923 K), our mechanical data indicate 3–4 orders of magnitude difference in strength between dry samples and specimens containing trace amounts of water.

High‐temperature creep of partially molten plagioclase aggregates

We have investigated the high-temperature creep of synthetic labradorite (An60Ab40) between 1323 K and 1523 K at atmospheric pressure and low stresses (2–65 MPa). Average grain size varies from 12 to 16 μm. Samples contained up to 12 vol % melt (An60Ab40 glass). Fourier transform infrared measurements indicate that trace amounts of water (∼0.1–0.15 wt % H2O) were incorporated during crystallization. This water was only partly released at high temperature and atmospheric pressure. At low stresses and temperatures, deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n∼1 and an activation energy of 365±25 kJ/mol (regime I). With increasing stress and temperature, mechanical data and microstructural observations indicate a transition to power law creep with n∼3 (regime II). This transition occurs in both partially molten and melt-free aggregates. In regime I the strain rate of samples containing as much as 12 vol % melt is between 3 and 5 times higher than in fully crystalline specimens. The activation energy is only slightly affected by the melt content between 0 and 10±2 vol %. Transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets. Grain boundaries are not wetted except surrounding large melt pockets. The melt distribution of undeformed and deformed samples is similar. The strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water. Both plagioclase compositions are slightly weaker than synthetic diopside.

An earthquake gap south of Istanbul

Over the last century the North Anatolian Fault Zone in Turkey has produced a remarkable sequence of large earthquakes. These events have now left an earthquake gap south of Istanbul and beneath the Marmara Sea, a gap that has not been filled for 250 years. Here we investigate the nature of the eastern end of this gap using microearthquakes recorded by seismographs primarily on the Princes Islands offshore Istanbul. This segment lies at the western terminus of the 1999 Mw7.4 Izmit earthquake. Starting from there, we identify a 30-km-long fault patch that is entirely aseismic down to a depth of 10 km. Our evidence indicates that this patch is locked and is therefore a potential nucleation point for another Marmara segment earthquake-a potential that has significant natural hazards implications for the roughly 13 million Istanbul residents immediately to its north.

Spatiotemporal changes, faulting regimes, and source parameters of induced seismicity: A case study from The Geysers geothermal field

The spatiotemporal, kinematic, and source characteristics of induced seismicity occurring at different fluid injection rates are investigated to determine the predominant physical mechanisms responsible for induced seismicity at the northwestern part of The Geysers geothermal field, California. We analyze a relocated hypocenter catalog from a seismicity cluster where significant variations of the stress tensor orientation were previously observed to correlate with injection rates. We find that these stress tensor orientation changes may be related to increased pore pressure and the corresponding changes in poroelastic stresses at reservoir depth. Seismic events during peak injections tend to occur at greater distances from the injection well, preferentially trending parallel to the maximum horizontal stress direction. In contrast, at lower injection rates the seismicity tends to align in a different direction which suggests the presence of a local fault. During peak injection intervals, the relative contribution of strike-slip faulting mechanisms increases. Furthermore, increases in fluid injection rates also coincide with a decrease in b values. Our observations suggest that regardless of the injection stage, most of the induced seismicity results from thermal fracturing of the reservoir rock. However, during peak injection intervals, the increase in pore pressure may likewise be responsible for the induced seismicity. By estimating the thermal and hydraulic diffusivities of the reservoir, we confirm that the characteristic diffusion length for pore pressure is much greater than the corresponding length scale for temperature and also more consistent with the spatial extent of seismicity observed during different injection rates.

Effect of quartz inclusions on plastic flow in marble

To investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks, we produced synthetic marbles with a grainsize of 5-25 μm, containing quartz of similar size, with volume fractions of 0, 5.1, and 20.4 vol.%, and between 3 to 9 % porosity. These rocks were deformed in triaxial compression at a strain-rate of 3 x 10 -5 s -1 , confining pressure of 200 MPa, and 600°C. Marbles containing 20.4 vol.% quartz were almost 5 times stronger than those containing no quartz. The work-hardening rate at given strain increased with quartz content. Marbles containing 20.4 vol.% quartz work-hardened up to 10 % strain whereas in single-phase samples work hardening was negligible beyond 3 % strain. The dislocation density in the calcite increased with increasing quartz content. The data are consistent with models for the strength of ceramic matrix composites, or with the hypothesis that strength is increased by decreasing porosity.

Effects of long‐term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field

The long-term temporal and spatial changes in statistical, source, and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (U.S.) are analyzed in relation to the field operations, fluid migration, and constraints on the maximum likely magnitude. Two injection wells, Prati-9 and Prati-29, located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7 year period were analyzed. The seismicity catalog was relocated, and the source characteristics including focal mechanisms and static source parameters were refined using first-motion polarity, spectral fitting, and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters (b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial, and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir.